docs: def _get_read_depths(cls, read_depths_file, sequence_name, position):
          '''Returns total read depth and depth of reads supporting alternative (if present)'''
          assert os.path.exists(read_depths_file)
          assert os.path.exists(read_depths_file + '.tbi')
          tbx = pysam.TabixFile(read_depths_file)
          try:
              rows = [x for x in tbx.fetch(sequence_name, position, position + 1)]
          except:
              return None
  
          if len(rows) > 1: # which happens with indels, mutiple lines for same base of reference
              test_rows = [x for x in rows if x.rstrip().split()[3] != '.']
              if len(test_rows) != 1:
                  rows = [rows[-1]]
              else:
                  rows = test_rows
  
          if len(rows) == 1:
              r, p, ref_base, alt_base, ref_counts, alt_counts = rows[0].rstrip().split()
              bases = ref_base if alt_base == '.' else ref_base + ',' + alt_base
              return bases, int(ref_counts), alt_counts
          else:
              return None